The proposed research focuses on the RNA capping reaction in trypanosomes. The investigator has purified the capping enzyme (guanylyltransferase) from Crithidia fasciculata and has cloned the corresponding genes from C. fasciculata and Trypanosoma brucei. The long-term goal of the proposed research is to understand the mechanism and the basis for the RNA specificity of the capping reaction of trypanosomatids. The investigator proposes three specific aims: (1) to undertake a structure-function analysis of the trypanosome capping enzyme; (2) to assess the consequences of genetic depletion of the capping enzyme; and (3) to search for cellular components that physically interact with the capping enzyme.